The voice is produce by the vibration into the larynx of the air coming from the lungs, vibration which is modified by resonances in the mouth and in the nose.
The voice is therefore determined by three systems:
Singing well involves the use and control of these three systems.
To have a powerful and stable voice, we must have a powerful and stable airflow. To do this, we must work on the muscles which are involves in this production, but also in the posture that would make the best use of the capacities of these muscles.
To breath out the air, we must begin by inspire it and fill the lungs. This is the role of the inspiring muscles the most important of which is the diaphragm. Then we contract the expiration muscles for blowing the air through the larynx. The inspiration and expiration muscles work in pairs and allow a fine control of the air flow. The inspiration muscles slow down the expiration muscles so that all the air is not blown at once. The control of the diaphragm is important both to inspire a large quantity of air and to control the flow during the singing.
When you are sitting or when we are stooped, our lungs can not be filled completely and therefore our ability to breathe is reduced. To sing well, we must stand and be in an upright position.
The head position has also some influence, the chin, pulled up or down, can interfere with air flow. It also causes the tongue muscles to tighten, which alters the sound production.
The vibration of the air is carried out by the vocal cords, which are types of flanges flesh inside of the larynx. The precise mechanism is described in the diagram below.
When we inspires, the vocal cords are open and the air pass freely through the larynx.
When we speak or sing we contract the muscles of the vocal cords which close the passage of the air, the air pressure increase, the vocal cords are forced to open and a breath of air escape producing a sound, the air pressure decrease and the vocal cords close again. This cycle repeats at a frequency which depends on the tension in the vocal cords and the air pressure. It's just the same mechanism that when you whistle, except that in the latter case, it is the lips that makes the sound.